Monolithic catalytic converters typically include an extruded ceramic honeycomb structure of a refractory material such as cordierite (2MgO-5SiO.sub.2 -2Al.sub.2 O.sub.3) or mullite, (3Al.sub.2 O.sub.3 -2SiO.sub.2). Formed metallic foil monolithic structures are also commonly used. This monolithic structure is wash coated with a thin layer of a catalyst carrier such as alumina or zirconium oxide of very high surface area. The high surface area carrier is usually impregnated uniformly throughout with a noble metal mixture such as platinum, palladium, rhodium.
After the monolithic structure and the catalyst are heated to the activation temperature of the catalyst, the noxious components of the automobile exhaust gas such as unburned hydrocarbons (UHC), carbon monoxide (CO) and nitrogen oxides (NOx) react at the catalyst site to form harmless gaseous products. Each catalyst material has a different activation temperature at which the rate of the catalyzed reaction increases from very low to very high levels. Therefore, it is desirable to design a monolithic converter for automobile emission controls which can heat up quickly to the activation temperatures of the catalyst materials or alternatively, which is characterized by a lower activation temperature. This is particularly important for alternative fuel applications, such as alcohol-containing fuels like methanol, since the alcohol-containing fuel generates undesirable aldehydes.
The use of an alcohol-containing fuel is problematic because the alcohols only partially combust in the combustion chamber, thereby resulting in the formation of these aldehydes. These aldehydes must be converted to harmless gaseous products, similarly to (as well as with) the unburned hydrocarbons and carbon monoxide. This conversion of the aldehydes may be accomplished in a conventional catalytic converter when the catalysts are heated to a sufficient temperature above the catalysts' activation temperature. Another and more significant problem is that the unburned alcohol-containing fuels from the combustion chamber may be only partially oxidized on the catalytic converter producing aldehydes. The formation of these aldehydes is more likely during the period when the catalyst temperature is in the transition region below its activation temperature.
An alternative mode for determining catalyst efficiency is to reference its "light-off" time, which is defined as that time period required for the catalyst to reach 50 percent efficiency in reacting with a particular noxious gas component. Obviously, a lower light-off time is more desirable. Therefore, in order to eliminate, or at least minimize, the formation of these undesirable aldehydes, it is necessary to provide a catalytic converter which is characterized by a low light-off time.
What is needed then is a monolithic catalytic converter for alcohol-containing fuel applications which minimizes the formation of aldehydes, such as by providing a catalytic converter characterized by a rapid heat-up and corresponding lower light-off time.